Tree View - Null parent_id when adding child test plan

[edoswaldgo]

Description of problem

Failed to add a child test plan when using the tree view.

Version or commit hash (if applicable)

v7.2.x

Steps to Reproduce

1. In View Test Plan, go to Tree View tab
2. Click Add child node to current plan
3. Specify the Test Plan ID to be added as child node, click Submit.

Actual results

Error 500 is encountered with null parent_id in request.

Expected results

Child node should be added successfully.

Additional info (Python traceback, logs, etc.)

File ".../tcms/testplans/views.py" in post
408.  parent_id = int(request.POST.get('parent_id'))

Exception Type: ValueError at /plan/update-parent/
Exception Value: invalid literal for int() with base 10: ''

[edoswaldgo]

Hi! I submitted a pull request at #1308 . Please help review and correct the change as needed. Thank you!